Hybrid electric vehicles (HEV) use different onboard energy sources, either individually or together, in order to optimize fuel economy. An HEV having a full hybrid powertrain can use either or both of an internal combustion engine and a high-voltage (HV) energy storage system (ESS) for propulsion. One or more HV motor/generator units (MGU) may alternately draw power from and deliver power to the ESS. By way of contrast, a mild HEV ordinarily cannot be propelled by electrical means, but nevertheless retains certain fuel saving powertrain design features of the full HEV. For example, a mild HEV is able to selectively shut off and restart its engine when the vehicle is stationary, thus reducing idle fuel consumption relative to conventional vehicle designs.